One-step process for the preparation of halide-free hydrophobic salts

ABSTRACT

This invention describes a one pot, single-step process for the preparation of halide-free hydrophobic salts comprising polyalkylated imidazolium cations and various anions in accordance with the following structure, where R 1  and R 3  represent the either the same or different alkyl groups, and R 2 , R 4 , and R 5  represent either hydrogen atoms, or the same or different alkyl group substituents; X represents a polyatomic anion that is the conjugate base of an acid. By simply mixing aqueous formaldehyde with an alkyl amine such as methylanune, ethylamine, n-propyl oriso-propylamine, or n-butyl-, iso-butyl, or t-butylamine, or by mixing aqueous formaldehyde with two alkyl amines (preferably one being methylamine, ethylamine, n-propyl- or iso-propylamine, or n-butyl-, isobutyl, or t-butylamine) and another being n-propyl- or isopropylaine, or n-butyl-, isbutyl, or t-butylamine), an acid (such as hexafluorophosphoric acid, trifluoroacetic acid, pentafluoropropionic, heptafluorobutyric acid, or the free acid of a bis(perfluoroalkylsulfonyprnide or tris(perfluoroalkylsulfonyl)methide as the source of the anion) and aqueous glyoxal solution, the hydrophobic ionic salts or mixtures thereof thus formed may be conveniently separated directly from the aqueous byproduct layer. Like the single cation hydrophobic salts, these mixed hydrophobic ionic liquids are non-flammable and manifest no detectable vapor pressure up to their decomposition temperature of greater than 300° C. We have also discovered that, surprisingly, ternary mixtures of dialkylated ionic liquids manifest higher ionic conductivities than a single ionic liquid of the mixture alone. This property benefits electrochemical power source applications such as batteries and capacitors. Furthermore, we have discovered that ternary mixtures of dialkylated ionic liquids absorb microwave radiation more efficiently than a single ionic liquid of the mixture alone. This property benefits microwave-induced synthetic reactions. Such physical and chemical properties make it possible to employ inexpensive mixtures of polyalkylated imidazolium cations in an advantageous manner as thermal transfer fluids, high temperature lubricants, and plasticizers, and as solvents in the areas of electrochemistry, synthetic chemistry, catalysis, and separations chemistry.

This application is a 371 of PCT/US02/01766 filed on Jan. 22, 2002.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of U.S. Provisional Application No.60/262,661 filed Jan. 22, 2001 entitled ONE-STEP PROCESS FOR PREPARATIONOF HYDROPHOBIC IONIC LIQUIDS, the whole of which is incorporated byreference herein.

FIELD OF THE INVENTION

This invention relates to a simple, one-step method of preparinghalide-free hydrophobic salts comprising polyalkylated imidazoliumcations and mixtures thereof using inexpensive reactants. Suchpolyalkylated imidazolium cations, when paired with polyatoniic anionsfor charge balance, offer tremendous potential in applications such asthermal transfer fluids, high temperature lubricants, and plasticizers,and as solvents in the areas of electrochemistry, synthetic chemistry,catalysis, and separations chemistry.

BACKGROUND OF THE INVENTION

Hydrophobic salts are a class of materials comprising electronicallydelocalized organic cations and polyatomic organic or inorganic anions.These salts, which typically possess melting points of less than 175°C., and more particularly, below 150° C., may also be liquid at roomtemperature and below.

In the 1940s Hurley and Wier first disclosed the preparation and use ofionic salts, which were fluid at room temperature, comprising theelectronically delocalized N-alkylpyridinium cation and haloaluniinateinorganic anions in a series of U.S. Pat. (Nos. 2,446,331, 2,446,339,and 2,446,350). However, the practical utility of thesehaloaluminate-based pyridinium salts is severely compromised by theirextremely high reactivity with trace amounts of water leading to theliberation of heat and toxic gases.

In the 1990s, air and water stable salts comprising the electronicallydelocalized 1,3-dialkylimidazolium cation and non-haloaluminate anions,e.g. BF₄ ⁻ and PF₆ ⁻ were prepared by Wilkes and coworkers (J Chem. Soc.Chem. Comm. 965 [1992]). However, these salts, because the alkyl groupson the imidazolium cation possessed less than three carbon atoms, have asignificant solubility in water and may not be described as“hydrophobic”. Ellis describes (WO 9618459) a process for making ionicliquids by reacting a solution of a lead salt of an anion desired in theionic liquid with 1,3-dialkylimidazolium halide salts, and separatingthe lead halide precipitate from an aqueous solution of the hydrophilicionic liquid product. Finally, in U.S. Pat. No. 5,182,405 Arduengodescribes a one-step process for the preparation of salts comprising1,3-disubstituted imidazolium cations and the conjugate base of an acid.These salts, which are highly hydrophilic, are prepared via reaction ofan alpha-dicarbonyl compound, an aldehyde (in toluene), an amine, and anacid comprising hydrogen halides, sulfric acid, or phosphoric acid.

The multi-step preparation of the more technologically usefulhydrophobic salts comprising 1,3-dialkylimidazolium cations and thebis[trifluoromethylsulfonyl]imide anion was disclosed in U.S. Pat. No.5,683,832 and in Inorg. Chem. 1168 (1996). U.S. Pat. No. 5,827,602describes a multi-step reaction route leading to a broad spectrum of airand water stable hydrophobic salts comprising 1,3-dialkylimidazoliumcations coupled with non-Lewis acid containing polyatomic anionspossessing a van der Waals volume of greater than 100 A³.

However, all of the known procedures for preparation of hydrophobicsalts minimally involve a two-step reaction sequence starting with anexpensive 1-alkyllimidazole followed by alkylation by an alkyl halide.In addition to the high cost of the alkylimidazole, halide impurities(such as Cl⁻, Br⁻, and I⁻) from alkyl halide starting materials are bothdifficult and time consuming to remove. The purification step employinga silver reagent to produce halide-free hydrophobic salts is veryexpensive and unsuitable as an industrial process. Therefore, a needexists to improve the process for the preparation of ionic salts thatare both hydrophobic and halide-free. The ability to inexpensivelyproduce industrial quantities of such salts will enable the introductionof these technologically useful materials into a host of cost-sensitivechemical and engineering applications.

BRIEF SUMMARY OF THE INVENTION

One object of this invention is to describe a novel process capable ofproducing hydrophobic salts inexpensively and halide-free. Anotherobject of this invention is to produce mixtures of low meltinghydrophobic salts inexpensively and halide-free. A further object ofthis invention is to demonstrate that mixtures of hydrophobic saltspossess hitherto unrealized advantages that make such mixtures moreuseful for various applications than a single hydrophobic salt alone.

This invention describes a one pot, single-step process for thepreparation of halide-free hydrophobic salts comprising polyalkylatedimidazolium cations and various anions in accordance with the followingstructure,

where R₁ and R₃ represent the either the same or different alkyl groups,and R₂, R₄, and R₅ represent either hydrogen atoms, or the same ordifferent alkyl group substituents; X⁻ represents a polyatomic anionthat is the conjugate base of an acid.

By simply mixing aqueous formaldehyde with an alkyl amine such asmethylamine, ethylamine, n-propyl- or iso-propylamine, or n-butyl-,iso-butyl, or t-butylamine, or by mixing aqueous formaldehyde with twoalkyl amines (preferably one being methylamine, ethylamine, n-propyl- oriso-propylamine, or n-butyl-, iso-butyl, or t-butylamine) and anotherbeing n-propyl- or iso-propylamine, or n-butyl-, iso-butyl, ort-butylamine), an acid (such as hexafluorophosphoric acid,trifluoroacetic acid, pentafluoropropionic, heptafluorobutyric acid,trifluorosulfonic acid, or the free acid of abis(perfluoroalkylsulfonyl)imide or tris(perfluoroalkylsulfonyl)methideas the source of the anion) and aqueous glyoxal solution, thehydrophobic salt thus formed may be conveniently separated directly fromthe aqueous byproduct layer.

Like the single cation hydrophobic salts, these mixed hydrophobic ionicliquids are non-flammable and manifest no detectable vapor pressure upto their decomposition temperature of greater than 300° C. We have alsodiscovered that, surprisingly, ternary mixtures of dialkylated ionicliquids manifest higher ionic conductivities than a single ionic liquidof the mixture alone. This property benefits electrochemical powersource applications such as batteries and capacitors. Furthermore, wehave discovered that ternary mixtures of dialkylated ionic liquidsabsorb microwave radiation more efficiently than a single ionic liquidof the mixture alone. This property benefits microwave-induced syntheticreactions. Such physical and chemical properties make it possible toemploy inexpensive mixtures of polyalkylated imidazolium cations in anadvantageous manner as thermal transfer fluids, high temperaturelubricants, and plasticizers, and as solvents in the areas ofelectrochemistry, synthetic chemistry, catalysis, and separationschemistry.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a plot of the temperature of various neat ionic liquidsirradiated with microwave energy as a function of time.

FIG. 2 shows a plot of the temperature of various ionic liquidsdissolved in propylene carbonate and irradiated with microwave energy asa function of time.

DETAILED DESCRIPTION OF THE INVENTION

Hydrophobic ionic salts based on polyalkylated imidazolium cations offertremendous potential in applications such as thermal transfer fluids,high temperature lubricants, and plasticizers, and as solvents in theareas of electrochemistry, synthetic chemistry, catalysis, andseparations chemistry. Huddleston and coworkers (Green Chemistry, 3, 156[2001]) have recently identified hydrophobic salts and hydrophobic ionicliquids as possessing superior physical and chemical properties overhydrophilic ionic liquids in respect to separations chemistry. Organicannulation reactions were found by Morrison and coworkers (Tet. Lett.,42, 6053 [2001]) to proceed with excellent yields in a recyclablehydrophobic ionic liquid solvent. Zhang and coworkers (PolymerPreprints, 42, 583 [2001]) have found that free-radical polymerizationreactions conducted in hydrophobic ionic liquids achieved highermolecular weights than those conducted in conventional organic solvents.McEwen and coworkers (J. Electrochem. Soc., 146, 1687 [1999]) havereported on the superiority of hydrophobic polyalkylated imidazoliumsalts as electrolytes for electrochemical double layer capacitors.

We have discovered that, surprisingly, ternary mixtures of dialkylatedionic liquids manifest higher ionic conductivities than a single ionicliquid of the mixture alone. Higher ionic conductivities allow anelectrochemical power source, such as a battery or capacitor, to delivermore power and also to enable low temperature applications. We have alsodiscovered that ternary mixtures of dialkylated ionic liquids absorbmicrowave radiation more efficiently than a single ionic liquid of themixture alone. In particular, we find that an inexpensive 2:1:1 mixtureof hydrophobic 1,3-dialkylated imidazolium hexafluorophosphates A, B,and C shown below reaches temperatures in excess of 150° C. in a matterof seconds. This property benefits microwave-induced synthetic reactionsby reducing reaction times.

Applications relying on hydrophobic ionic salts would greatly benefitfrom a simple, inexpensive synthetic route to these novel materials.Hydrophobic ionic salts have previously been synthesized by a multi-steproute relying on imidazole, alkylated imidazoles, and alkyl halidestarting materials. Starting materials based on the imidazole ring areintrinsically expensive, and the use of alkyl halides in the reactionscheme results in Cl⁻, Br⁻, and I⁻ byproducts that require extensivepurification to eliminate. This adds significantly to the overall costof the desired hydrophobic ionic salts.

The present invention provides a method for the convenient andinexpensive preparation of halide-free hydrophobic salts comprising oneor more polyalkylated imidzolium cations and various polyatomic organicor inorganic anions. While Arduengo in U.S. Pat. No. 5,182,405 teaches asimilar one-step route to imidazolium salts, it must be pointed out thatsuch imidazolium salts are extremely hydrophilic in nature, and moredifficult to purify than the hydrophobic ionic salts. The term“hydrophilic salt” is intended to mean a salt that is highly soluble inwater. The term “hydrophobic salt” is intended to mean a salt thatpossesses very limited, if any, solubility in water. Some of thehydrophobic ionic salts may be termed ionic liquids. The term “ionicliquid” is intended to mean liquids that are comprised entirely of ions,and when in the neat form at a pressure of 1 atmosphere have a meltingpoint of 150° C. or less.

We have developed a simple, one-step synthesis of the hydrophobic ionicsalts shown below, where the R₁ and R₃ substituents of the polyalkylatedimidazolium cation are C₁₋₂₀ alkyl groups,

where “C₁₋₂₀ alkyl” is intended to mean a linear, cyclic, or branchedhydrocarbon group having from 1 to 20 carbon atoms, such as methyl,ethyl, n-propyl, iso-propyl, cyclopropyl, n-butyl, sec-butyl, t-butylgroup, cyclobutyl, pentyl, cyclopentyl, hexadecyl, heptadecyl,octadecyl, or nonadecyl. The R₂, R₄, and R₅ substituents representeither hydrogen atoms, or the same C₁₋₂₀ alkyl group substituents asdefined for R₁ and R₃ above.

The counter ion “X” refers to either polyatomic inorganic or polyatomicorganic anions that are the conjugate base of an acid “HX”. Illustrativeexamples of such anions are PF₆ ⁻, AsF₆ ⁻, SbF₆ ⁻, C_(n)F_(2n+1)CO₂ ⁻,C_(n)F_(2n+1)SO₃ ⁻, where n=1 to 10 carbon atoms in either straight orbranched chains, (C_(n)F_(2n+1)SO₂)₂N⁻, (C_(n)F_(2n+1)SO₂)₃C⁻, where n=1to 5 carbon atoms in either straight or branched chains, and(C_(n)F_(2n+1))PF₅ ⁻, (C_(n)F_(2n+1))₂PF₄ ⁻, (C_(n)F_(2n+1))₃PF₃ ⁻, and(C_(n)F_(2n+1))₄PF₂ ⁻, where n=1 to 5 carbon atoms in either straight orbranched chains.

Hydrophobic polyalkylated imidazolium salts are obtained in one stepfrom inexpensive and commercially available aldehydes, alkylamines,α-dicarbonyl compounds, glyoxal, and acids (HX) comprising polyatomicanions (X⁻).

Aldehydes comprising either hydrogen or alkyl group substituents of from1–10 carbon atoms are preferred. Preferred alkylamines possess from 1–10carbon atoms. When two or more amines are employed in the process, it ispreferred that at least one amine comprise from 4–10 carbon atoms.α-Dicarbonyl compounds useful in the process preferably comprise eitherhydrogen and/or alkyl group substituents of from 1–10 carbon atoms.

Acids (HX) must comprise polyatomic counterions (X⁻) and are limited tothose counterions that result in hydrophobic rather than hydrophilicionic salts. Preferred acids are HPF₆, CF₃CO₂H, CF₃SO₃H, (CF₃SO₂)₂NH,(CF₃SO₂)₃CH, and (CF₃)₂PF₄H.

The one-step process to halide-free hydrophobic ionic salts can becarried out between from about −20° C. to about 150° C., and mostpreferably between 0° C. to 75° C., preferably with stirring. Reactiontimes may vary from 1 to 72 hours depending upon the reactiontemperature and more typically from 2 to 24 hours.

The hydrophobic ionic salts typically form as a solid precipitate, or astwo liquid phases in the reaction vessel: a bottom layer comprising thehydrophobic ionic liquid (or ionic liquid mixture) immiscible withwater, and an aqueous top layer. The desired reaction products may beeasily separated from the undesired reaction byproducts by common means,for example by filtration in the case of solid products or decantationin the case of liquid products. It is the hydrophobic nature of theionic salts of this invention that allows them to be readily obtained inhigh purity thus precluding the expense incurred by the need forextensive purification.

It has been discovered in this invention that salts composed of amixture of three 1,3-dialkylimidazolium cations (i.e., R₁≠R₃,) and ananion, or of certain symmetrical 1,3-dialkylimidazolium cations, such as1,3-di-n-propylimidazolium or 1,3-di-n-butylimidazolium (i.e., R₁=R₃ )and a suitable anion are hydrophobic ionic liquids at ambienttemperature. The mixture of compounds can be represented by thefollowing general structures,

where R₁ and R₃ are alkyl radicals comprising a different number ofcarbon atoms, preferably R₁ being a methyl, ethyl, n-propyl, iso-propyl,n-butyl, sec-butyl, or t-butyl group and R₃ being a n-propyl,iso-propyl, n-butyl, sec-butyl, or t-butyl group, and R₂, R₄, and R₅represent either hydrogen atoms, or the same or different alkyl groupsubstituents as in R₁ and R₃. The mixture of imidazolium salts has apreferable ratio of A:B:C=2:1:1. By using a slightly higher proportionof n-propyl- or iso-propylamine, or n-butyl-, iso-butyl-, ort-butylamine, hydrophobic ionic liquids having a higher ratio of A and Ccan be produced. In cases where R₁ and R₃ are identical, they shouldpreferably be n-propyl, iso-propyl, n-butyl, iso-butyl, or t-butylgroups. The counter ion “X” refers to either polyatomic inorganic orpolyatomic organic anions that are the conjugate base of an acid “HX”.Illustrative examples of such anions are PF₆ ⁻, AsF₆ ⁻, SbF₆ ⁻,C_(n)F_(2n+1)CO₂ ⁻, C_(n)F_(2n+1)SO₃ ⁻, where n=1 to 10 carbon atoms ineither straight or branched chains, (C_(n)F_(2n+1)SO₂)₂N⁻,(C_(n)F_(2n+1)SO₂)₃C⁻, where n=1 to 5 carbon atoms in either straight orbranched chains, and (C_(n)F_(2n+1))PF₅ ⁻, (C_(n)F_(2n+1))₂PF₄ ⁻,(C_(n)F_(2n+1))₃PF₃ ⁻, and (C_(n)F_(2n+1))₄PF₂ ⁻, where n=1 to 5 carbonatoms in either straight or branched chains.

The following examples describe the preparation of hydrophobic saltsaccording to this invention, as well as some of their physical andchemical properties that make them suitable for use in a broad spectrumof chemical and engineering applications. These examples are intended tofurther illustrate, but not limit, the invention.

EXAMPLE 1 Preparation of 1,3-Diethylimidazolium Hexafluorophosphate

Aqueous formaldehyde (15 ml of 37 wt % formaldehyde; 0.20 mol) wasmagnetically stirred in a 250 ml Erlenmeyer flask immersed in an icebath. Aqueous ethylamine (32 ml of 70 wt % ethylamine; 0.40 mol) wasadded in drops to the cold formaldehyde. The solution was stirred 15minutes and aqueous hexafluorophosphoric acid (30 ml of 60 wt % HPF₆;0.20 mol) was added in small portions from a plastic syringe. A whiteprecipitate formed during the addition, but re-dissolved to give a clearsolution, which was stirred ˜15 minutes before aqueous glyoxal (23 ml of40 wt % glyoxal; 0.20 mol) was added drop-wise. The solution was stirredovernight, during which time the ice bath warmed to room temperature. Awhite precipitate formed. The mixture was stirred approximately another30 hours at room temperature and was chilled in the refrigerator. Theprecipitate was collected by suction filtration, washed with cold water,and dried under vacuum at room temperature to give 21.4 g (79 mmol; 40%)of crude product, mp 70–72 ° C. Re-crystallization from methanol/diethylether afforded 16.8 g (62 mmol; 31%) of analytically pure material, mp71–73 ° C.

IR (KBr) 3178, 3120, 1568, 1167, 843 cm⁻¹

¹H NMR (CD₃CN) 8.44 (s, 1, H₂), 7.39 (d, 2, H₄, H₅), 4.16 (q, 4), 1.46(t, 6) ppm

Anal. Calcd for C₇H₁₃F₆N₂P: C, 31.12; H, 4.86; N, 10.37.

-   -   Found: C, 31.36; H, 4.83; N, 10.40.

EXAMPLE 2 Preparation of 1,3-Diethylimidazolium Hexafluoroarsenate

Aqueous ethylamine (32 ml of 70 wt % ethylamine; 0.40 mol) was added indrops to ice-cold, magnetically stirred aqueous formaldehyde (15 ml of37 wt % formaldehyde; 0.20 mol) in a 250 ml Erlenmeyer flask immersed inan ice bath. Aqueous hexafluoroarsenic acid (59 g of 65 wt % AsF₆H; 0.20mol) was added in drops, causing the separation of a white precipitate.The mixture was stirred 10 minutes before the addition of aqueousglyoxal (23 ml of 40 wt % glyoxal; 0.20 mol). The mixture was stirredovernight while the ice bath warmed to room temperature spontaineously.The two-phase reaction mixture consisted of an upper aqueous phase and agolden-yellow, viscous liquid. In the process of decanting thesupernatant aqueous layer, the lower viscous liquid phase crystallizedexothermically. The crystalline product was collected by suctionfiltration and washed with water. The product was dissolved in warmmethanol and re-crystallized by addition of diethyl ether. The whitecrystalline product amounted to 42.14 g (134 mmol; 67%), mp 61–62° C.

IR (KBr) 3176, 3120, 1568, 1165, 848, 756, 696 cm⁻¹

¹H NMR (CD₃CN) 8.44 (s, 1, H₂), 7.39 (d, 2, H₄, H₅), 4.16 (q, 4), 1.46(t, 6) ppm

EXAMPLE 3 Preparation of 1,3,4-Trimethylimidazolium Hexafluorophosphate

Aqueous methylamine (35 ml of 40 wt % methylamine; 0.40 mol) was addedin drops to ice-cold, magnetically stirred aqueous formaldehyde (15 mlof 37 wt % formaldehyde; 0.20 mol) in a 250 ml Erlenmeyer flask immersedin an ice bath. After 10 minutes, aqueous hexafluorophosphoric acid (30ml of 60 wt % HPF₆; 0.20 mol) was added in small portions from a plasticsyringe. Finally, aqueous pyruvic aldehyde (33 ml of 40 wt % pyruvicaldehyde; 0.20 mol) was added in drops from a pipet. The reactionsolution took on the yellow color of the pyruvic aldehyde. The solutionwas stirred while the ice bath was allowed to warm spontaneously. Withina few hours, a precipitate had formed and the color of the mixture haddarkened. The mixture was stirred overnight at room temperature, bywhich time the mixture was dark red and contained a considerableproportion of solid. The precipitate was collected by suctionfiltration, washed with cold water, and filtered giving ˜30 g of crudeproduct. The crude product was dissolved in warm methanol (˜100 ml), thesolution was filtered, and the filtrate was allowed to cool. Crystalsformed on standing overnight. The crystals were collected, washed withdiethyl ether and dried under vacuum at room temperature. The yield was21.76 g (85 mmol; 42%) of pale yellow crystals, mp 84–86 ° C.

IR (KBr) 3186, 3157, 3128, 1616, 1587, 1456, 1169, 850 cm⁻¹

¹H NMR (CD₃CN) 8.26 (s, 1, H₂), 7.07 (s, 1, H₅), 3.76 (s, 3), 3.68 (s,3), 2.25 (d, 3) ppm

Anal. Calcd for C₆H₁₁F₆N₂P: C, 28.13; H, 4.34; N, 10.94.

-   -   Found: C, 28.35; H, 4.24; N, 10.93.

EXAMPLE 4 Preparation of 1,2,3,4-TetramethylimidazoliumHexafluorophosphate

Aqueous methylamine (35 ml of 40 w/o methylamine; 0.40 mol) was added ina drop wise fashion to an ice-cold, magnetically stirred solution ofacetaldehyde (12 ml; 0.20 mol) in 350 ml of water in a 500 ml Erlenmeyerflask in an ice bath. Aqueous hexafluorophosphoric acid (30 ml of 60 wt% HPF₆; 0.20 mol) was added slowly in small portions from a plasticsyringe. Aqueous pyruvic aldehyde (33 ml of 40 wt % pyruvic aldehyde;0.20 mol) was then added drop wise from a pipet. The solution wasstirred overnight while the ice bath was allowed to warm spontaneously.The precipitate, which formed overnight was collected by suctionfiltration and washed with cold water. The crude product wasre-crystallized from methanol to give 11.46 g (42 mmol; 21%) of1,2,3,4-tetramethylimidazolium hexafluorophosphate as white crystals, mp151–153 ° C.

IR (KBr) 3145, 2970, 1630, 1554, 1446, 1209, 847, 787 cm⁻¹

¹H NMR (CD₃CN) 6.97 (d, 1, H₅), 3.65 (s, 3), 3.55 (s, 3), 2.47 (s, 3),2.23 (d, 3) ppm

Anal. Calcd for C₇H₁₃F₆N₂P: C, 31.12; H, 4.86; N, 10.37.

-   -   Found: C, 31.37; H, 4.91; N, 10.47.

EXAMPLE 5 Preparation of 1,3,4,5-TetramethylimidazoliumHexafluorophosphate

Aqueous methylamine (35 ml of 40 wt % methylamine; 0.40 mol) was addedin a drop wise manner to ice-cold, magnetically stirred aqueousformaldehyde (15 ml of 37 wt % formaldehyde; 0.20 mol) in a 250 mlErlenmeyer flask in an ice bath. Aqueous hexafluorophosphoric acid (30ml of 60 wt % HPF₆; 0.20 mol) was added in small portions from a plasticsyringe, followed by drop wise addition of 2,3-butanedione (17.6 ml;17.3 g; 0.20 mol). The reaction solution became yellow and viscous; itwas stirred overnight while the ice bath warmed to room temperature. Themixture was chilled in the refrigerator. The precipitate was collectedby suction filtration, washed with cold water, and dried under vacuum atroom temperature to give 27.0 g (100 mmol; 50%) of crude product, mp151–155 ° C. Re-crystallization from methanol afforded analytically purematerial (23.0 g; 85 mmol; 43%), mp 156–158 ° C.

IR (KBr) 3190, 3128, 1585, 1454, 1213, 843 cm⁻¹

¹H NMR (CD₃CN) 8.22 (s, 1, H₂), 3.66 (s, 6), 2.19 (s, 6) ppm

Anal. Calcd for C₇H₁₃F₆N₂P: C, 31.12; H, 4.86; N, 10.37.

-   -   Found: C, 31.39; H, 4.94; N, 10.44.

EXAMPLE 6 Preparation of 1,2,3,4,5-PentamethylnimdazoliumHexafluorophosphate

Aqueous methylamine (36 ml of 40 wt % methylamine; 0.42 mol) was addedin a drop wise fashion to a cold, magnetically stirred solution ofacetaldehyde (12 ml; 9.4 g; 0.21 mol) in water (˜350 ml) in a 500 mlErlenmeyer flask in an ice bath. The solution was stirred for 10 minutesand aqueous hexafluorophosphoric acid (30 ml of 60 wt % HPF₆; 0.20 mol)was slowly added from a plastic syringe. The solution was stirred 10minutes before 2,3-butanedione (18 ml; 17.7 g; 0.21 mol) was added dropwise. The solution took on a pale yellow color and within 30 minutes, aprecipitate formed. The mixture was stirred for two days at roomtemperature. The precipitate was collected by suction filtration, washedwith water, and allowed to air dry. The crude product amounted to 27.9 g(98 mmol; 49%) of yellow crystals. Re-crystallization from methanolafforded pale yellow crystals, mp 165–167 ° C.

IR (KBr) 1651, 1549, 1450, 843 cm⁻¹

¹H NMR (CD₃CN) 3.54 (s, 6), 2.47 (s, 3), 2.18 (s, 6) ppm

EXAMPLE 7 Preparation of 1,3-Di-n-Propylimidazolium Hexafluorophosphate

Aqueous formaldehyde (37%) (8 ml, 0.10 mol)) was chilled in a 125 mlErlenmeyer flask immersed in an ice-water bath. n-Propylamine (16.6 ml,0.20 mol was added drop wise with stirring. Aqueous hexafluorophosphoricacid (60%) (15 ml, 0.10 mol) was added in small portions from a plasticsyringe. Aqueous glyoxal (40%) (12 ml, 0.10 mol) was added drop wise andthe mixture was allowed to stir overnight at room temperature yieldingtwo liquid layers. The mixture was heated to 50° C. for 3 hours, and thelower layer separated and dried overnight on a rotary evaporator at 55°C. to give 18 g (62% yield) of a light orange liquid. Proton NMR showedthat this hydrophobic salt is comprised of 1,3-di-n-propylimidazoliumhexafluorophosphate, liquid at room temperature.

EXAMPLE 8 Preparation of a Mixture of 2:1:1 ratio of1-n-Butyl-3-Ethylimidazolium, 1,3-Diethylimidazolium, and1,3-Di-n-Butylnimdazolium Hexafluorophosphates

Aqueous formaldehyde (37%) (15 ml, 0.20 mol)) was chilled in a 250 mlErlenmeyer flask immersed in an ice-water bath. Aqueous ethylamine (16ml, 0.20 mol) was added drop wise with stirring, followed byn-butylamine (20 ml, 0.20 mol). Aqueous hexafluorophosphoric acid (60%)(30 ml, 0.20 mol) was added in small portions from a plastic syringe.Finally, aqueous glyoxal (40%) (23 ml, 0.20 mol) was added drop wise andthe mixture was allowed to stir for two days at room temperatureyielding two liquid layers. The lower layer containing the dialkylatedimidazolium salts was separated from the aqueous layer via a separatoryfunnel and washed with 100 ml of cold water affording 42 g of a faintyellow liquid (70% yield). Proton NMR confirmed the presence of amixture of hydrophobic substituted imidazolium hexafluorophosphatesalts, liquid at room temperature, incorporating a 2:1:1 ratio of1-n-butyl-3-ethylimidazolium, 1,3-diethylimidazolium and 1,3-di-n-butylhexafluorophosphates.

EXAMPLE 9 Preparation of a Mixture of 2:1:1 ratio of1-n-Butyl-3-Methylimidazolium, 1,3-Dimethylimidazolium, and1,3-Di-n-Butylimidazolium Hexafluorophosphates

Aqueous formaldehyde (37%) (17.6 ml, 0.225 mol)) was chilled in a 250 mlErlenmeyer flask immersed in an ice-water bath. n-Butylamine (22.5 ml,0.225 mol was added drop wise with stirring, followed by aqueousmethylamine (40%) (19.5 ml, 0.225 mol)). Aqueous hexafluorophosphoricacid (60%) (34 ml, 0.225 mol) was added in small portions from a plasticsyringe. Aqueous glyoxal (40%) (27 ml, 0.225 mol) was added drop wiseand the mixture was allowed to stir overnight at room temperatureyielding two liquid phases. The mixture was heated to 50° C. overnight,cooled, and the lower layer separated and lyophilized to give 60 g (95%yield) of a light brown liquid. Proton NMR and mass spectrometry showedthat this hydrophobic ionic liquid is comprised of a mixture of 2:1:1ratio of 1-n-butyl-3-methylimidazolium, 1,3-dimethylirnidazolium and1,3-di-n-butyl hexafluorophosphates.

EXAMPLE 10 Preparation of a Mixture of a 5:1:4 Ratio of1-n-Butyl-3-Methylimidazolium, 1,3-Dimethylimidazolium, and1,3-Di-n-Butylimldazolium Hexafluorophosphates

Aqueous formaldehyde (37%) (15.6 ml, 0.20 mol)) was chilled in a 250 mlErlenmeyer flask immersed in an ice-water bath. n-Butylamine (22.5 ml,0.225 mol was added drop wise with stirring, followed by aqueousmethylamine (40%) (15.1 ml, 0.175 mol). Aqueous hexafluorophosphoricacid (60%) (30 ml, 0.20 mol) was added in small portions from a plasticsyringe. Aqueous glyoxal (40%) (24 ml, 0.20 mol) was added drop wise andthe mixture was allowed to stir overnight at room temperature-yieldingtwo liquid layers. The mixture was heated to 50° C. overnight and thelower layer separated via a separatory funnel and dried on a rotaryevaporator at 55° C. overnight to give 56 g (99% yield) of a lightorange liquid. Proton NMR and mass spectrometry showed that thishydrophobic ionic liquid is comprised of a mixture of 5:4:1 ratio of1-n-butyl-3-methylimidazolium, 1,3-dimethylimidazolium and1,3-di-n-butyl hexafluorophosphates.

EXAMPLE 11 Ionic Conductivity and Viscosity

We have measured the room temperature conductivities and viscosities of1-n-butyl-3-ethylimidazolium hexafluorophosphate (BEIPF₆),1,3-Diethlyimidazolium hexafluorophosphate (DEIPF₆), and the 2:1:1mixture of 1-n-butyl-3-ethylimidazolium, 1,3-diethyliridazolium and1,3-di-n-butylimidazolium hexafluorophosphates. BEIPF₆ was prepared from1-ethylimidazole, n-butylbromide (both from Aldrich) and KPF₆ (Strem),and purified in accordance with the multi-step route described in U.S.Pat. No. 5,827,602. DEIPF₆ (which is a solid at room temperature) wasprepared from 1-ethylimidazole, ethylbromide (both from Aldrich) andKPF₆ (Strem), and purified in accordance with the multi-step routedescribed in U.S. Pat. No. 5,827,602. We found that neat BEIPF₆ is twiceas conductive as the 2:1:1 mixture as shown in Table 1. This finding isreasonable given that the viscosity data reveal that the 2:1:1 mixtureis more than twice as viscous as the BEIPF₆.

We have also found, surprisingly, that a 2:1:1 mixture of1-n-butyl-3-ethylimidazolium, 1,3-diethylimidazolium and1,3-di-n-butylimidazolium hexafluorophosphates when dissolved inacetonitrile manifest higher room temperature conductivities than eitherBEIPF₆ or DEIPF₆ dissolved in acetonitrile (AN) alone (Table 1). Thisresult is counterintuitive since one skilled in the art would expectthat the 2:1:1 mixture (which comprises the large, poorly mobile1,3-di-n-butylimidazolium cation) would be less conductive than asolution of the smaller diethylimidazolium and1-n-butyl-3-ethylimidazolium cations dissolved in AN. Indeed, ourviscosity measurements on samples of the 2:1:1 mixture, DEIPF₆, andBEIPF₆ dissolved in AN confirms this hypothesis: the viscosities of the2.0M BEIPF₆ and 2.0M DEIPF₆ in AN are slightly lower than the viscosityof 2.0M 2:1:1 mixture in AN. This, in turn, implies that the ionicconductivities of the BEIPF₆/AN and DEIPF₆/AN solutions should behigher. We believe that there may be some synergistic solvation effectsamong the three polyalkylated imidazolium cations and the AN solventthat unexpectedly enhance the ionic conductivity of the solution.

TABLE 1 Specific Conductivities and Viscosities of Various Ionic Liquidsat 22° C. Ionic Liquid conductivity, mS/cm viscosity, cP 2:1:1 mixture(neat) 1.6 251 BEIPF₆ (neat) 3.3 112 1.5 M 2:1:1 mixture/AN 42.0 — 1.5 MBEIPF₆/AN 41.0 — 2.0 M 2:1:1 mixture/AN 43.0 1.4 2.0 M BEIPF₆/AN 38.91.3 2.0 M DEIPF₆/AN 41.4 1.2 3.0 M 2:1:1 mixture/AN 25.9 — 3.0 MBEIPF₆/AN 23.0 —

Because the sensitivity of the conductivity measurements was ±0.1 mS/cm,these data clearly demonstrate that one can achieve higher ionicconductivities over various concentrations by employing a mixture ofhydrophobic ionic liquids instead of a single hydrophobic ionic liquid.Hydrophobic ionic liquid mixtures, when dissolved in a suitable solvent,would find use as high conductivity electrolytes in electrochemicalpower sources. Preferably these solvents are linear and cyclic organiccarbonates, ethers, ketones, esters, formates, nitrites, nitroderivatives, amides, sulfones, sulfolanes, sulfonamides, partiallyhalogenated hydrocarbons, polymers and combinations thereof.

EXAMPLE 12 Response to Microwave Radiation

The thermal stability and lack of a vapor pressure make ionic liquidsmaterials ideal for microwave assisted organic synthesis as described byWestman in WO 00/72956. We have found that a 2:1:1 mixture of1-n-butyl-3-ethylimidazolium, 1,3-diethylimidazolium and1,3-di-n-butylimidazolium hexafluorophosphates absorb microwaveradiation at a faster rate than 1-n-butyl-3-ethylimidazoliumhexafluorophosphate (EIPF₆) hydrophobic ionic liquid alone. Samples of 4ml each of neat BEIPF₆, the neat 2:1:1 mixture, and water were subjectedto microwave radiation provided by a small kitchen unit. The temperatureof each sample was measured by a digital thermocouple as a function ofvarious irradiation times. As shown in FIG. 1, after 7 seconds ofirradiation the water warms up from room temperature to 65° C., whilethe 2:1:1 mixture reaches a temperature of 160° C. compared to 115° C.for the single BEIPF₆ ionic liquid. Therefore, the neat 2:1:1 mixture ofthree hydrophobic ionic liquids is a more efficient medium for theabsorption of thermal energy induced by microwave radiation.

We have also found that when a 2:1:1 mixture of1-n-butyl-3-ethylimidazolium, 1,3-diethylimidazolium and1,3-di-n-butylimidazolium hexafluorophosphates is dissolved in anorganic solvent such as propylene carbonate, the mixture absorbsmicrowave radiation at a faster rate than either1-n-butyl-3-ethylimidazolium hexafluorophosphate (BEIPF₆) or1,3-diethylimidazolium hexafluorophosphate (DEIPF₆) dissolved alone inpropylene carbonate (PC). FIG. 2 shows that the 2:1:1 mixture reaches atemperature of 220° C. after 7 seconds while the BEIPF₆ and BEIPF₆ reacha temperature of 195° C. Therefore, the 2:1:1 mixture of threehydrophobic ionic liquids dissolved in PC is a more efficient medium forthe absorption of thermal energy induced by microwave radiation.

The improved rate of microwave absorption is observable both of theionic liquids are used neat or if they are in solution. Solvents usablefor these solutions are preferably organic solvents and include linearand cyclic organic carbonates, ethers, ketones, esters, formates,nitrites (particularly acetonitrile), nitro derivatives, amides,sulfones, sulfolanes, sulfonamides, partially halogenated hydrocarbons,polymers and combinations thereof.

Those with expertise in this technology will recognize variations thatare consistent with the disclosure herein.

1. A process for making one or more hydrophobic ionic salts of thestructure

wherein R₁ and R₃ each is an alkyl moiety, each of R₂, R₄, and R₅ ishydrogen or said alkyl moiety, and X⁻ is an anion, selected from thegroup consisting of polyatomic inorganic and polyatomic organic anions,said process comprising the steps of: a. mixing an aldehyde havingeither hydrogen or alkyl group substituents with one or more alkylamines comprising an alkyl moiety desired for said hydrophobic ionicsalts to form a mixture; b. adding the acid form of one or more anionsdesired for said hydrophobic ionic salts to said mixture; c. adding anaqueous solution of an alpha-dicarbonyl compound to said mixture; d.allowing the components of said mixture to react; and e. separating saidhydrophobic ionic salts from said mixture.
 2. The process of claim 1wherein said hydrophobic ionic salts are ionic liquids.
 3. The processof claim 2 wherein a single hydrophobic ionic liquid is made.
 4. Theprocess of claim 2 wherein two or more hydrophobic ionic liquids aremade.
 5. The process of claim 4 wherein said hydrophobid ionic liquidshave improved ionic conductivity.
 6. The process of claim 4 wherein saidhydrophobic ionic liquids have improved absorption of microwave energy.7. The process of claim 1 wherein said alkyl moieties are the same. 8.The process of claim 1 wherein said alkyl moieties are different.
 9. Theprocess of claim 8 wherein at least one of said alkyl moieties comprises4 to 10 carbon atoms.
 10. The process of claim 1 wherein said anion isselected from the group consisting of PF₆ ⁻, AsF₆ ⁻, SbF₆ ⁻,C_(n)F_(2n+1)CO₂, C_(n)F_(2n+1)SO₃ ⁻, where n=1 to 10 carbon atoms ineither straight or branched chains; (C_(n)F_(2n+1)SO₂)₂N⁻,(C_(n)F_(2n+1)SO₂)₃C⁻, where n=1 to 5 carbon atoms in either straight orbranched chains; and (C_(n)F_(2n+1))PF₅ ⁻, (C_(n)F_(2n+1))₂PF₄ ⁻,(C_(n)F_(2n+1))₃PF₃ ⁻, and (C_(n)F_(2n+1))₄PF₂ ⁻, where n=1 to 5 carbonatoms in either straight or branched chains.
 11. The process of claim 10wherein said anion is PF₆ ⁻.
 12. The process of claim 1 wherein, in step(a), said alkyl amines are one or more C₁₋₂₀ alkyl amines, where C₁₋₂₀alkyl is a linear, cyclic or branched hydrocarbon group having from 1 to20 carbon atoms.
 13. The process of claim 12 wherein the alkyl moietiesof said alkyl amines are selected from the group consisting of methyl,ethyl, n-propyl, iso-propyl, cyclopropyl, n-butyl, sec-butyl, t-butyl,cyclobutyl, pentyl, cyclopentyl, hexadecyl, heptadecyl, octadecyl andnonadecyl.
 14. The process of claim 2 wherein, in step (a), said alkylamines are one or more C₁₋₂₀ alkyl amines, where C₁₋₂₀ alkyl is alinear, cyclic or branched hydrocarbon group having from 1 to 20 carbonatoms.
 15. The process of claim 14 wherein the alkyl moieties of saidalkyl amines are selected from the group consisting of methyl, ethyl,n-propyl, iso-propyl, cyclopropyl, n-butyl, sec-butyl, t-butyl,cyclobutyl, pentyl, cyclopentyl, hexadecyl, heptadecyl, octadecyl andnonadecyl.
 16. The process of claim 1, wherein, in step (a), said alkylgroup substituents comprise from 1 to 10 carbon atoms.
 17. The processof claim 16, wherein, in step (a), said aldehyde is aqueousformaldehyde.
 18. The process of claim 17, wherein, in step (a), saidaldehyde is acetaldehyde.